1. Field of the Invention
This invention relates to a process for the heat treatment of a steel rail. More particularly, it relates to a process for cooling steel rails in heated condition whereby there is obtained a steel rail having improved hardness. This invention is particularly directed to a process for the preparation of steel rails characterized by high hardness, which steels do not contain substantial quantities of alloying elements. This invention is particularly concerned with the preparation of steel rails involving the quenching of steel rails in heated condition employing water at a temperature of at least 80.degree. C.
2. Discussion of the Prior Art
Present-day standard rails employed in the railroad industry are naturally hard rails which, after rolling, are cooled on a cooling bed in motionless air. Alternatively, they can be cooled utilizing a delaying action in order to avoid flaking in cooling pits or carriages. See "Stahl and Eisen", 81, pages 1253-1263 (1961).
In accordance with the "Technical Delivery Conditions of the International Railway Union UIC 860-V", these rails should have a minimum tensile strength of 686 or 882 N/mm.sup.2, respectively. As the reference or guide analyses in Table 1 below show in respect of Examples 1 to 3, the carbon and manganese are the main components which impart strength. The structure of these rails is pearlitic-ferrite for Example 1 and pearlitic for Examples 2 and 3.
Because of increasing traffic employing the rails and partly in conjunction with the fact of higher axle loads being experienced, there is a need for higher strength rails. Specifically, there is a need to provide steel rails having a minimum tensile strength of 1080 N/mm.sup.2 which corresponds to a minimum tensile strength of 110 kp/mm.sup.2. See "Krupp Technical Reports 32" (1974) Volume 1.
Unfortunately, this kine of strength can no longer be obtained employing the naturally hard carbon-manganese steels. It has been required to include alloying elements in the steel such as chromium, vanadium and molybdenum. As seen in Table 1, Example 4 below, such a steel having a silicon content can be employed which will raise the rail hardness to at least 882 N/mm.sup.2 minimum tensile strength where the steel additionally contains approximately 1% chromium. However, even this improved tensile strength is insufficient in many circumstances, for the rails should be characterized by a minimum tensile strength of at least 1080 N/mm.sup.2.
The use of relatively high alloy contents and additional alloying elements adds to the expense of the rails and involves certain special welding techniques. Additionally, since in continuously welded railway tracks more than 50% of all of the rail damage and fractures occurs at the weld (see "Eisenbahntechnische Rundschau" 22 (1973), Volume 6) the suitability of such a rail of such a composition is in doubt owing to its high alloy content. With a view to advantageous suitability for welding, it is desirable to provide a steel rail having high strength values which has as low an alloy content as possible. It is the alloy content which provides problems in the welding of the steels together and which increases the likelihood of rail damage at the welds. Thus, attention has been directed to various rail heat treatments as an answer to the problem of providing higher tensile strength steel rails.
Methods which have been normally been used for the heat treatment of rails involve the cooling of the heated rails, after welding, with some delay in static air or in pits. Thereafter, they can be treated in one of two basic manners. In one process following straightening, they are austenitized in batches in a furnace and then quenched in oil and furnacetempered. In such a case the entire cross section is treated (full quench and temper treatment). In another manner following straightening, the rails are heated with burners or inductively in a continuous process and then the rails are quenched with sprays and tempered. See "Stahl and Eisen" 90 (1970) pages 922, 928. This latter continuous throughflow method is used almost exclusively for the heat treatment of the rail head or partial regions of the rail head, i.e., the entire cross section of the rail is not so treated. The rail web and the rail base are left in the as-rolled state.
In the heat treatment involving the quenching in oil, a fine pearlitic structure is produced. Continuous throughflow methods in some cases work towards a pearlitic structure and to some extent towards a quenched or tempered structure. FIG. 1 of the accompanying drawings shows the hardening curves with respect to the distance from the running surface of a heat treated American (U.S.) rail which has been subjected to full quench and temper treatment, flame hardening and quench and temper treatment using the inductive method. An important difference lies in the fact that with quench and temper treatment, the hardness only decreases gradually towards the interior of the rail head and in all cross-sectional regions is higher than in the as-rolled state. For the flame-hardened and inductively hardened rails, the decrease in hardness is initially more abrupt, the hardness curve passing through a valley which is below the hardness for the as-rolled state. If there is not sufficiently deep heat treatment, there is a risk that with greater hardness in the running surface region below the running surface in the region of the high shearing stresses, there will be inadequate strength values. Obviously, these methods are characterized by certain disadvantages involving substantial additional expense for the renewed heating of the rails. In addition, these methods are not suitable for heat treatment on a large industrial scale and thus, cannot be used adequately as cooling means for the high output of a rail rolling mill.
Earlier recommended heat treatment for rails from rolling heat advocated the cooling of the rails with accelerated cooling employing short-duration immersion of the rail head in a bath of water or cooling by spraying the rail head with atomized water, steam or damp compressed air until the red heat disappeared. In such a case, the rail head and the rail base had to be protected before cooling. Such a process has not been adapted successfully on a large industrial scale. Details of such process are reported in the "3rd International Rail Conference of 8-12 September 1935, Hungarian Association for Material Testing Budapest, 1936." In addition to the obvious disadvantages in the earlier processes recommended, processing difficulties and uncertainties in respect of heat treatment provided by variations in the initial temperature of the steel, the quantity of the water and the duration of hardening presented obstacles to the acceptable utilization of such a process in production.
In addition to the other known methods (again as reported in the "3rd International Rail Conference Budapest") a process for the treatment of a rail head was proposed involving intermittent hardening by repeated short-duration dipping in the lifting and lowering hardening bath until the red heat of the steel disappeared (Nueves Maisons method). Alternatively, the rail head can be held in the water bath without interruption to 2/3 to 3/4 of its height (Maximilianshutte method). Both methods were technically quite involved and have been abandoned.
In a Polish proposal (again see the "3rd International Rail Conference Budapest") rails were conducted, standing or lying, at any desired speed through a heat treatment installation, the surfaces intended for heat treatment being sprinkled with atomized water. Other surfaces were protected from accelerated cooling. Hardening of the rail to beyond 300.degree. Brinell was regarded as undesirable because of the danger of flaking. For reasons of process technique, this method was not very suitable for processing rails at high output rate.
It therefore became desirable to provide a commercially and technically feasible and sound process for the heat treatment of steel rails. More particularly, it became desirable to provide a process for the heat treatment of rails wherein the rails would acquire a superior tensile strength, particularly at the rail head. More especially, it became desirable to provide such a process which could improve the tensile strength of the rail head without the use of substantial quantities of alloying elements.